imtrobin:
There is TrueCrypt Explorer which is a portable without needing admin rights. It works but needs an update.

http://sourceforge.net/projects/tcexplorer/

MerleOne:
And, as I said earlier, Safehouse Explorer also proposes container encryption without admin rights. The only trick I know is to get the full application, not the installer, which tries to install a driver.

wurst:
AFAIK TC Explorer needs admin rights as well.

Just a week ago the excellent c't magazine introduced a tool for true usb-stick encryption with open source tools, working in Windows/Linux/OSX. This method does not need admin rights and works cross-platform. Check the (google translated) website here: http://translate.google.com/translate?u=http%3A%2F%2Fwww.withopf.com%2Ftools%2Fsecurstick%2F&langpair=de|enOr Original in German here: http://www.withopf.com/tools/securstick/